A landscape prime

Indeed 15mm sounds good for landscape, but may in fact be too wide for most 'scapes. If you can get only one I'd go for the 21mm. I'd like to get it one day too as the focal length seems like the perfect walk around lens for APS-C.

@OP: I have the 21 ltd and the Sigma 10-20, I do not have the 15 because whenever I use the 10-20 I end up at 10 so I suspect I would be frustrated by the 15. The 10-20 is collosal but I do carry it in my bag. If pockets only then I take the 21.

I would have thought the 15 was too wide for 'standard' landscapes where the sky and clouds behave themselves. The 21 is an excellent street lens being small and also having quick shift which is very handy.

On balance I'd get the 21 but I'm not sure if you really need it given you have the 10-20 which I find to be an excellent lens once you learn how to use it which I expect you have.

As a matter of interest are you a member of any club near Worcester?

Mike

When it comes to lenses so wide, a lot depends on curvelinear distortion, ie, the bending of straight lines in the image.

Now I have only owned or played with a couple of the lenses in question, and I can say thus:-

Sigma 10-20mm - Bad distortion, certainly until 15mm, but can be used to effect.
Pentax 16-45mm - Rather good. There is some, but not that you would really notice.
Pentax 18-55mm (kit) - bad wide, but zoom in, its good.
Pentax - 21mm Ltd - Exceptional, hardly saw any at all.

I don't know about the 12-24 nor the 15mm Ltd or indeed the 14mm, perhaps others can advise.

But I have voted for the 21mm as I think it will be more useful as a lens, even if it does not have that superwide ability of the 15mm.

Me? I still want both!

If you're talking about barrel distortion, the 21mm has LOTS for a lens of such a moderately wide focal length. Of course it's easily corrected. The 15 is much straighter despite being so much wider.

I used to have both the 21mm and thge 15. I found that I rarely used the 21mm probably because it lay within the range of my general purpose zoom.

On the other hand the 15mm is a delightful lens which is very sharp when stopped down a little and as Will comments above it has low distortin and striking resistance to flare. It also usefully extends the range of my usual zoom (currently the DA 18-135) at the wide end and as it is small and light takes up little room in the camera bag or can easily be carried in a pocket. The wide angle zooms tend to be much bigger and heavier.

As usual it is largely a matter of personal preference.

Personally I'd find the 15mm a bit too long, but then it depends on the style of shot required.

When I'm using the Sigma 10-20mm I can't recall ever using a length other than 10mm, if there was a good 10mm prime I'd simply get that. The distortion is easily corrected in Bibble or Lightroom, but most times I don't actually bother. In practice I use the 10-20 as a 10mm fixed lens.

Using the Tamron 17-50 it seems to stay stuck on 17, which is usefully wider than the kit 18-55, but I still find that for the type of pictures I'm aiming for the field of view at 17 isn't wide enough.

I'd certainly like to see what the 15mm is like in IR - different lenses can give results completely different from their visible light characteristics. In particular they can be plagued by IR flare that is completely absent in normal shots.